1. Field of the Invention
The present invention is directed to powder coating compositions and coatings produced therefrom which have flat finishes.
2. Description of the Prior Art
Polyurethane stoving lacquers are known and described in U.S. Pat. Nos. 4,055,550; 4,252,923; 4,369,301; and 4,480,008. The coating compositions may be processed as a solution or as a powder and are based on a polyhydroxyl component and a polyisocyanate component wherein the isocyanate groups are completely or partially blocked with a blocking agent to prevent premature reaction of the components. A cross-linking reaction to form the coating only occurs at elevated temperatures which are sufficient to release the blocking agent.
Regardless of whether the polyhydroxyl component is a polyester polyol or a polyacrylate polyol, the coatings prepared therefrom generally have a high gloss. In order to produce coatings with a flat or matt finish, it is necessary to add flatting agents to the coating composition. However, the incorporation of these flatting agents often reduces other desirable and necessary properties of the coating.
U.S. Pat. No. 4,480,008 is directed to the incorporation of dihydroxy carboxylic acids during the preparation of the blocked polyisocyanate component in order to provide coatings having a flat finish.
It is an object of the present invention to provide stoving compositions which may be used to prepare coatings having a flat finish. It is an additional object of the present invention to provide stoving compositions which may be used to prepare coatings which retain the desirable and necessary properties of stoving lacquers. These objects may be achieved in accordance with the present invention by the incorporation of certain components during the preparation of the blocked polyisocyanate as described hereinafter.
U.S. Pat. No. 4,369,301 is directed to the production of stoving compositions based on a blocked polyisocyanate which has been prepared by reacting an organic polyisocyanate with compounds containing hydroxyl groups and structural units corresponding to the formula EQU --O--CO--NH--NH--CO--O--
and/or the formula EQU H.sub.2 N--NH--CO--O--
Coatings prepared from the polyisocyanate component disclosed therein possess improved resistance to yellowing. There is no disclosure in this reference that the disclosed structural units provide coatings having a flat finish when the blocked isocyanates are reacted with polyacrylate polyols.